So what are your thoughts on using the narrow v-bar grip and trying to get more ROM out of the exercise by almost letting the stack get all the way back down at the end of each rep. I know there's more chance to abuse this and start swinging yoru body around to lift more weight, but the stretch in the lats when doing it this way at least give the illusion that it is more effective.

If you're flexing at the hip (which is fine up to a point, see my comment response on the article), it's not doing anything to stretch the lats. It may seem like it but you're not changing anything about the length-tension relationship of the lats by leaning forwards at the hip.

That is not me in the video but I have similar form. Underhand grip row hits maybe 5 inches above the belly button. I'm wondering if this within the normal range of deviation (possibly having to do with the location of the cable attachment itself) or if it is possibly more of a technical problem/imbalance/tight upper back/whatever.

I've tried to row to the belly button and it just doesn't 'fit' if that makes any sense. The shoulder blades just don't go that low even when scooping them down and back.

Look at his forearms and how they are tilted above the horizontal. He's also leaning back slightly which changes where it will hit

I find that it's hard to force the bar to stay lower so forearms stay horizontal. Is it not related to how the bar attaches to the cable stack relative to where the seat is (how far off the ground)? I've tried rowing to the belly button but it always creeps up a tick.